U.S. Pat. No. 6,269,891 (hereby incorporated by reference) discloses a system for drilling and logging of 10 a wellbore formed in an earth formation, wherein a logging tool can be lowered in the wellbore from inside a drill string through a drill bit at the lower end of the drill string.
The known system comprises a drill bit including a bit body provided with a passageway for the logging tool, and a closure element for the passageway in the form of an insert section at the bit face. The bit body is attachable to a tubular drill string at a drill-string side of the bit body, and the passageway extends during normal operation in a well from an opening at the drill-string side to the well exterior of the bit body. The closure element comprises a bit-connecting means in the form of a primary latching device for selectively connecting the closure element to the bit body, so as to selectively close the passageway. The latching device can be manipulated by an auxiliary tool that forms the downstream part of a logging tool string.
The drill bit of the known system can be used for drilling operations when the closure element is connected to the bit body. When it is desired to log the formation drilling operation is stopped, the drill bit is pulled up an appropriate distance to expose the desired interval and the logging tool string with the auxiliary tool at its lower end is lowered through the drill string into the passageway. The tool-connecting means is connected to the closure element, and, simultaneously, the bit-connecting means is operated so as to release the closure element from the bit body. Then, the logging tool with the closure element attached to its lower end can be lowered into the wellbore ahead of the drill bit from where logging can be performed. After logging has been completed, the logging tool string can be pulled back into the drill string, so that the closure element is re-connected to the bit body and the auxiliary tool is simultaneously disconnected from the closure element.
U.S. Pat. No. 7,296,639 (EP1588016), U.S. Pat. No. 7,140,454 (EP 1404941), U.S. Pat. No. 7,281,592 (W003/010410), and U.S. Pat. No. 7,287,609 (EP 1570156) are all hereby incorporated by reference. These references disclose other embodiments of systems and methods for performing an operation in a wellbore ahead of a drill bit, wherein a tool is passed through a passageway in the bit body, connected to the closure element, and passed further to an external position in the borehole ahead of the bit body with the closure element connected to the lower end of the tool, at which external position the tool can be used to perform the operation.
U.S. Pat. No. 4,384,627 discloses drilling bit with a retractable pair of knives, which are the only cutting elements of the drilling bit. US 2006/0021801 A1 discloses a retrievable center bit for use with a tubular including an inner bore and a drill shoe cutter mounted thereon. The center bit is locked axially and rotationally to the tubular, so that during drilling operation the drilling forces are transmitted via the locking assembly. U.S. Pat. No. 4,760,888 discloses a coring bit with retrievable cutters. In some operations it is undesirable to have the closure element attached to the lower end of the tool.
Applicant's co-pending application PCT/EP02/07533, published as W003/004825, which is hereby incorporated by reference, discloses a drill bit assembly where the closure element is secured using a passage tool, which provides a pathway for the operating tool into the wellbore that is not obstructed by the closure element. The tubular upper part of the passage tool remains at least partly in the passageway of the bit body and serves by itself as a passageway (with a reduced internal diameter) for the operating tool, from an upstream position in the drill string to a port at its lower end, through which the operating tool can pass.
In many situations, it would be desirable to be able to retrieve the closure element to surface, having full diameter opening and enabling another tool to be inserted through the passageway into the bit body. Thus there is a need to develop a robust system which allows the closure element to be retrieved and reinserted through the passageway so another tool may be inserted unobstructed. In addition, there is a need for a method which allows coring to be done with the drill bit.